Hey Hoot,
Thanks.
I'll hook it up to an oscilloscope with and without a capacitor.
I read further here:
http://www.kpsec.freeuk.com/powersup.htm
And found a calculation for the capacitance required. It seems at 5A (the max current supply for my PSU) I need 3600uF, or 1800uF at 2.5A (the continuous current for my pumps).
So I guess 2200uF will be perfect.